This invention relates to automated control of laser output characteristics, and more particularly to a method and apparatus for automatic initialization and alignment of laser cavity length to attain and maintain single longitudinal mode (SLM) laser operation over a wide range of selected operating frequencies.
There are many techniques in the prior art for controlling the operating frequency of a laser and the bandwidth or spread of laser output frequencies. There are two distinctly different physical phenomena that control the laser operating frequency and the frequency distribution of its output. The first phenomena relies upon physical dispersion, such as angular changes in wavelength pointing as light passes through a prism or is reflected off of a grating, or a variety of other techniques that alter pointing polarization, or phase as a function of wavelength. The second phenomena is the characteristic of the laser oscillator that limits effective oscillation at frequencies corresponding to wavelengths that are an integer multiple of the round trip path length within the laser oscillator cavity, or n.lambda.=2L, wherein n is the integer multiple, .lambda. is the wavelength and L is the laser oscillator cavity length.
Both of these phenomena are present in wavelength tunable lasers. The laser output is thus a convolution of the two phenomena. The wavelength dispersive optics are controlled by positional or induced field tuning, while the longitudinal mode distribution is controlled by changing only cavity length.
Recent advances in laser designs have made possible the selection of one longitudinal mode with wavelength dispersive optical elements. The procedures for attaining SLM operation according to the prior art involves alignment of the optical components of the laser cavity followed by alignment of the optically dispersive elements to select wavelength range followed finally by the adjustment of the laser cavity length. In most cases these adjustments are performed manually and the mechanical integrity of the device is relied upon to maintain alignment for an extended period of time.
Cavity length adjustments are typically very sensitive and require some sort of active control to maintain constant cavity length. Many different active feedback loops are available to lock the cavity length to a set value. These feedback loops rely upon physical phenomena that are coupled to the cavity length such as etalon fringes or beam pointing. In laser cavities where a wavelength dispersive element is involved, the output pointing is coupled to the longitudinal mode composition.
Yet, the procedure for attaining single longitudinal mode tracking according to the prior art is still to perform manual cavity length adjustments, followed by startup of an active feedback loop. This manual adjustment has been necessary according to the prior art in order to adjust the laser cavity length.